[caffe2] fix test error in video_input_op_test (#39382)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/39382
Test Plan: buck test caffe2/caffe2/python/operator_test:video_input_op_test
Reviewed By: dutran
Differential Revision: D21832355
fbshipit-source-id: 47b1b0610b9600437fe1ed317d5af47d624767fb
